Heretofore, both Rockeed and portless types have been known as typical types of a master cylinder used in a hydraulic circuit for vehicles.
A Rockeed type master cylinder is disclosed in, for example, Japanese Utility Model Laid-Open No. 61-59156 (1986). In this example, when a brake pedal is trod on, a piston is moved through a push rod, so that a pressure chamber is disconnected from a reservoir tank and comes into a sealed state, at a time when a piston cup has passed a compensating port. Then, the hydraulic pressure in the pressure chamber is increased while reducing the volume thereof.
In such a type master cylinder, therefore, by setting a distance between an inoperative position of the piston and the compensating port as small as possible, the hydraulic pressure in the pressure chamber can be increased as soon as the pedal is trod on and gradually at the beginning, as shown in FIGS. 3 and 4, in cooperation with the compensating port in the form of a restriction hole. This results in advantages of making small a lost stroke of the pedal as well as improving a control feeling and hydraulic operation.
In this type master cylinder, however, when the piston cup passes the compensating port, it is expanded while being subject to a hydraulic pressure in the pressure chamber with its part fretted into the compensating port. Thus, there is a fear that the piston cup would heavily be damaged and a liquid sealing function could not be maintained for a long period.
A portless type master cylinder has succeeded in solving the problem of the above type master cylinder and is disclosed in, for example, Japanese Utility Model No. 61-73467 (1986).
This type master cylinder has no compensating port, and a piston rod is fitted in a bore of the cylinder body such that it is normally urged by a return spring toward a push rod and liquid-tightly slidable within the bore. The piston includes a valve mechanism which valve is closed to seal the master cylinder when the master cylinder is in its operative position, and opened to communicate a pressure chamber and a supply chamber communicating with a reservoir when it is in its inoperative position, thereby balancing hydraulic pressures in both the chambers.
In the valve mechanism, generally, a piston has formed on the pressure chamber side a larger-diameter hole communicating with the supply chamber side through a fine hold at the center, and a valve rod having a valve body at its one end is loosely fitted in the fine hole and normally urged by a spring force toward the push rod, so that the valve body can be seated on and departed from a valve seat provided in the fine hole. Seating and departing of the valve body on and from the valve seat is effected as follows. When the brake is released, for example, the valve body is moved together with the piston toward the push rod while being seated on the valve seat by a valve spring force. Just before the piston reaches a final return position, the end of the valve rod abuts against a stop pin fixed to the cylinder body and inserted in a slit, the slit being lengthwisely formed in the piston, to thereby stop the valve rod at that position against the valve spring force.
At this time, since the piston continues its return movement, the valve body is now departed from the valve seat so that pressurized liquid in the pressure chamber is allowed to pass into the supply chamber.
Trading on the brake pedal causes the piston to be moved into the pressure chamber side through the push rod. At the beginning of this movement of the piston, the end of the valve rod remains abutted against the stop pin by virtue of the valve spring force. But the valve body starts to move together with the piston at the same time upon the valve body seating on the valve seat, to thereby increase the hydraulic pressure in the pressure chamber.
Accordingly, as will be understood from the above, the piston cup moves merely in sliding contact with the internal bore surface of the cylinder body in the foregoing portless type master cylinder, so the piston cup will most likely not be damaged and its liquid sealing function can be held for a long period.
In the portless type master cylinder, however, since the valve body formed of an elastic material such as rubber is deformed and extended toward the valve seat in the piston hole while being subject to the hydraulic pressure in the pressure chamber, the piston body is less easily departed from the valve seat when the piston is returned, resulting in a drawback that the brake drags. To prevent such a drawback, the length of the valve rod must be relatively increased to ensure a spacing between the valve body and the valve seat when the valve is open, taking into account the extended amount of the valve body.
The increased length of valve rod means that the time necessary for the valve body seating the valve seat is prolonged upon tread-on of the brake.
FIG. 3 and FIG. 4, which is an enlarged view of a part of FIG. 3, graphically show the magnitude of output versus a pedal stroke in Rockeed type and portless type master cylinders. As will be seen from these figures, the latter type is delayed in its starting of the output compared with the former type. This delay is consistently maintained over the entire stroke. In particular, it will be also found from FIG. 4 that the Rockeed type master cylinder has its output gradually increased at the beginning of the pedal tread-on stroke, while the portless type master cylinder has its output drastically raised up at an initial moment and then gradually increased.
This means that the portless type has a larger lost stroke than the Rockeed type, and it also accompanies the problem of a less comfortable feeling due to the rising shock.